1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. In particular, the present invention relates to a semiconductor device using a nonvolatile memory such as a NOR flash memory and a NAND flash memory and a manufacturing method of the semiconductor device.
2. Description of the Related Art
Semiconductor memory devices (semiconductor memories) using semiconductor elements are roughly classified into a volatile memory that loses its stored data when supply of power is stopped and a nonvolatile memory that can hold its stored data even when supply of power is stopped.
As a typical example of a volatile memory, a dynamic random access memory (DRAM) can be given. A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is stored in a capacitor. Each memory cell of a DRAM includes only one transistor and one capacitor. Thus, cost per storage capacity is low and which is favorable in terms of high integration.
When data is read out, charge of a capacitor is lost in a DRAM. For this reason, writing operation is needed every time data is read. In addition, in the state where the transistor is not selected, charge is lost due to leakage current or the like, so that a data storage period is short. Thus, another writing operation (refresh operation) is needed at predetermined intervals, and it is difficult to reduce power consumption sufficiently. Further, since stored data is lost when supply of power stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
As another example of a volatile memory, a static random access memory (SRAM) can be given. An SRAM remains stored data with a circuit such as a flip flop. For this reason, refresh operation is not needed and in view of this point, an SRAM is advantageous as compared to a DRAM. However, cost per storage capacity is high because a circuit such as a flip flop is used. As in the case of a DRAM, when supply of power stops, stored data is lost.
As typical examples of a nonvolatile memory, a NOR flash memory and a NAND flash memory can be given. A flash memory includes a memory transistor having a floating gate and a tunnel insulating film between a gate electrode and a channel formation region. Charge is introduced and held into the floating gate through the tunnel insulating film, so that data is stored. Consequently, a flash memory has an advantage that data can be kept extremely for a long period and refresh operation is not needed (for example, see Patent Document 1).
[Reference]
[Patent Document 1] Japanese Published Patent Application No. S57-105889